Multivibrator circuit



April 9 D. ENG STROM 2,418,826

MULTIVIBRATOR CIRCUIT Filed Jan. 22, 1943 INVENTOR O. 0. [N65 TROM ATTORNEY Patented Apr. 15, 1947 MULTIVIBRATOR CIRCUIT Olle I). Engstrom, East Williston, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 22, 1943, Serial No. 473,189

4 Claims.

This invention relates to an improvement in multivibrator circuits. It is of value at any oscillation frequency which the multivibrator may be used to produce but is particularly important when this frequency is lower than ten cycles per second.

The multivibrator circuit known to the prior art comprises a pair of thermionic vacuum tubes commonly referred to, one as an amplifier tube,-

the other as a trigger tube. Ordinarily the anode of the amplifier tube is conductively or capacitatively coupled to the control grid of the trigger tube, the control grid of the former being at the same time capacitatively coupled to the anode of the latter. The cathode circuit of the trigger tube comprises a condenser shunted by a constant current discharge tube of variable conductance or by a Variable resistance. When such a multivibrator is in operation the cathode condenser is alternately charged and discharged whereby sawtooth voltage oscillations are generated across its terminals. The frequency is determined by the selected conductance of the discharge tube or of the resist nce shunting the condenser. This sawtooth voltage is available to a subsequent circuit connected in parallel with the condenser, for example, the horizontal deflection plates of a cathode ray oscilloscope. Circuits of the type in question are disclosed, for example, in United States Patents 1,934,322, granted November '7, 1933, to W. O. Osbon. and 2,114,938, granted April 19, 1938, to O. S. Puckle, and are described in general terms in an article entitled Hard value scan ning circuits, by G. Parr, Televis on and Short- Wave World. August 1938, page 475.

The cap-ecitative feedback from the anode of the trigger tubeto the control grid of the amplifier tube effects the transfer between these ele-- ments of voltage impulses but involves the time constant of an R.-C. circuit comprising the feedback condenser and the customary grid leak of the amplifier tube, The charging .time of the feedback condenser circuit is not a negligible elimination of the condenser coupling the anode of the trigger tube to the control. grid of the amplifier tube. In the present invention this is accomplished by replacing the usual capacitative feedback by direct current feedback.

It is therefore an object of the invention to provide an improved multivibrator circuit.

Another object of the invention is to provide a multivibrator circuit whereby oscillating voltages may be generated having an amplitude independent of the oscillation frequency;

Finally, it is an object of the invention to provide a multivibrator circuit comprising an amplifier tube and a trigger tube wherein an auxiliary vacuum tube is used to couple the anode of the trigger tube to the control grid of the amplifier tube.

A'feature of the invention is the provision of a resistor in the anode circuit of the above-mentioned auxiliary tube across which appear, with amplification, variations in anode voltage of the trigger tube which are directly transferred to the control grid of the amplifier tube.

The invention will be understood from the following description read with reference to the accompanying drawing the single figure of which is a multivibrator circuit constituting a preferred embodiment of the invention.

Referring to the figure, Vi is the amplifier tube, suitably a GAC'I. In tube Vi, control grid is connected to ground through grid leak 2; cathode 4 and suppressor grid- 5 are connected together and biased positively to ground by 12-volt battery 3; screen grid 6 and anode l are supplied with positive potential from -Volt battery 8, grid 6 directly and anode I through resistor RI. Trigger tube V2 is suitably a 6L6. Its control grid 8 is conductively coupled, as shown, to the junction of anode of VI and resistor RB. Screen grid 9 and anode Iil of V2 are supplied from 300-V0lt battery ii, grid 9 directly and anode it through resistor R2. Cathode I2 of, V2 is capacitativeli coupled to ground through condenser C across the terminals of which appear the voltage oscillations enerated. in the operation of the, circuit.

In the multivibrator circuits of the prior art anode ill of V2 is coupled through a condenser to control grid 1 of Vt and condenser C is shunted either by a variable resistor or by a constant current discharge tube of variable conductance. In the circuit herein described anode H3 is coupled to grid 1 through the circuit of auxiliary tube V3, preferably a 605. In tube V3 cathode i3 is coupled to ground through. resistor R3, control grid it is negatively biased by 8-volt battery l5,

the positive terminal of which is connected to.

cathode l3, and anode I6 is conductively coupled to the junction of anode ID of V2 and resistor R2. Conductor I! connects directly cathode 13 of tube V3 and control grid I of tube VI. Tube V3 thus provides coupling between anode ill of V2 and control grid l of VI, replacing the condenser heretofore used to couple these elements.

Shunting condenser C is a discharge circuit including tube V4, suitably a 6A0? like tube VI.

Anode l8 of V4 is connected to cathode l2 of V2 at the junction of the latter with condenser C. Cathode I9 is connected to ground through a fixed resistor 20 in series with resistor 2i and 150-volt battery 22, the positive terminal of which is grounded and connected also to screen grid 23 of V4. Control grid 24 and suppressor grid 25 are together connected to the negative terminal of battery 22. passed by suitable condensers, not shown. Cathode heater supplies are likewise to be understood. Where linearity of sweep is unimportant a variable resistor may replace the discharge circuit just described.

In the operation of the above circuit saw-tooth voltage oscillations are generated across condenser C of frequency determined by the conductance of tube V4, which may be regulated by the adjustment of tap 21 on resistor 21 together with the capacity ofcondenser C. This capacity is conveniently 32 microfarads for frequencies from one-tenth to one cycle per second and 3.2 microfarads for frequencies from one to ten cycles per second. The voltage oscillations so generated are available to an external circuit across output terminals ill and 02, connected respectively to cathode 12 of V2 and to ground. The range of variation of voltage across condenser C is approximately 75 volts from a maximum of 150 volts to a minimum of 75 volts, approximately.

The novelty in the operation of the circuit will now be particularly described. For this purpose it is appropriate to tabulate the values found suitable for the various elements not already de- Consider the instant in the cycle, when trigger tube V2 is blocked, amplifier tube Vi is conducting and condenser C is charged to its maximum potential of approximately 150 volts. Under these conditions the voltage on anode H) of V2 and anode iii of V3 is a maximum. Tube V3 is conducting and by reason of the voltage drop across resistor R3 cathode I3 is approxi mately volts positive to ground. This potential of cathode i3 is impressed through conductor I! on control grid 1 of tube VI in opposition to the effective 12-volt negative bias from battery 3. Control grid 1 is thus about 2 volts negative to ground corresponding to an anode current IP, in tube VI, of about 10 mils.

Control grid 8 of tube V2 at this time is about 80 volts negative to ground, this potential arising from the voltage of battery 8, less the voltage drop acrossR] algebraically added to the negative bias derived from condenser C. The net voltage is -IPR1=80 volts. Now condenser C Batteries 8, II and 22 are bydischarges through the discharge circuit including tube V4 and the negative voltage on grid 8 is progressively reduced. As the voltage of condenser C approaches its minimum value of '75 volts, V2 begins to conduct and concurrently the voltage on anodes i0 and it decreases. The falling anode voltage of tube V3 corresponds to a decreasing space current in that tube which means a reduction in the voltage drop across R3 which through conductor ll opposes the negative bias of battery 3. The circuit elements of tube V3 are chosen such that this tube is operating on the steep portion of its IPEP characteristic. This being so, the falling anode voltage of tube V3 results in a rapidly increasing negative potential on grid I. The instant is speedily reached when the space current of V2 reaches saturation, at which instant the voltage drop across R3 is about 2 volts and grid I is 10 volts negative to ground. VI is now blocked and condenser C is being recharged by the space current in tube V2. When condenser C acquires its full voltage V2 blocks and the conductivity of Vi is regained, whereupon the described cycle is repeated.

It will be clear that the rapidity of trigger action is now controlled by the steepness of the IPEP characteristic of auxiliary tube V3 instead of by the much less steep discharge current of the usual coupling condenser and amplifier tube grid'leak which circuits of the prior art depend upon. It is obvious that'other types of auxiliary tubes than the 6C5 may be employed, and that by changes in the circuit elements easily made by one skilled in the art the auxiliary tube and the operation involved in the present invention may be applied to improve any form of multi vibrator. The steepness of the IPEP characteris-. tic of the auxiliary tube effects amplification of the triggering variation in potential of the anode of the trigger tube. By reason of this steepness the variationin voltage drop across R3 may be made several times the variation in voltage of anodes l0 and IS. The triggering interval required to convert tube Vi from the conducting to the blocking condition is made as short as desired with the result that the extremes of voltage reached by condenser C are not afiected by this interval whatever the frequency of recurrence of charge and discharge of the condenser.- Therefore the saw-tooth voltage output of the multivibrator is constant and this is obviously true whether the condenser charging interval is the retrace period, as in the circuit described, 0 is the sweep period of the multivibrator.

What is claimed is:

1. A multivibrator circuit including an amplifier tube and a trigger tube, each of said tubes having at least a cathode, a control grid and an anode, power supplies for said tubes, conductive interacting tubes having at least a cathode, a

control grid. and an anode, power supplies for. said tubes, conductive couplingbetween ground and the cathode of one of said tubes, capacitative coupling between ground and the cathode of the other of said tubes, conductive coupling between ground and the control grid of said one tube, coupling between the anode of said one tube and the control grid of said other tube, conductive coupling including a source of constant voltage between ground and the anode of said other tube, and means for rendering the amplitude of said voltage oscillations independent of frequency 1 including an auxiliary thermionic tube in circuit between the control grid of said one tube and the anode of said other tube.

3. A multivibrator circuit for the generation of saw-tooth voltage oscillations of variable frequency and constant amplitude comprising a first, a second and a third vacuum tube, each of said tubes having at least a cathode, a control grid and an anode, power supply for said tubes, conductive coupling between ground and the cathode of said first tube, conductive coupling between the anode of said first tube and the control grid of said second tube, capacitative coupling between ground and the cathode of said second tube, a discharge circuit in shunt with said capacitative coupling including means for controlling the frequency of said oscillations, conductive coupling between the anode of said second tube and the anode of said third tube, conductive coupling including a source of constant voltage between the control grid and the oathode of said third tube, conductive coupling between ground and the cathode of said third tube and conductive coupling between the cathode of said third tube and the control grid of said first tube.

4. A multivibrator circuit as described in claim 3 wherein said conductive coupling between ground and the cathode of said third tube is a resistor in parallel connection between ground and. the control grid of said first tube.

OLLE D. ENGSTROM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,114,938 Puckle Apr. 19, 1938 2,155,210 Young Apr. 18, 1939 2,265,290

Knick Dec. 6, 1941 

